Always on receiver with offset correction for implant to implant communication in an implantable medical system

ABSTRACT

Disclosed herein are implantable medical devices (IMDs) including a receiver and a battery, and methods for use therewith. The receiver includes first and second differential amplifiers, each of which monitors for a predetermined signal within a frequency range and drains power from the battery while enabled, and while not enabled drains substantially no power from the battery. To remove undesirable input offset voltages, each of the differential amplifiers, while enabled, is selectively put into an offset correction phase during which time the predetermined signal is not detectable by the differential amplifier. At any given time at least one of the first and second differential amplifiers is enabled without being in the offset correction phase so that at least one of the differential amplifiers is always monitoring for the predetermined signal. In this manner, the receiver is never blind to signals, including the predetermined signals, sent by another IMD.

FIELD OF TECHNOLOGY

Embodiments described herein generally relate to methods and systems forcommunication between implantable medical devices.

BACKGROUND

Implantable medical devices and systems often rely on propercommunications to operate correctly. For example, in a dual chamberpacemaker system, implant-to-implant (i2i) communications are criticalfor proper synchronization and operation of the system. Such a systemcan utilize conductive communication, whereby i2i communication signalsare received and transmitted using the same electrodes that are used forsensing and/or delivery of pacing therapy. Where conductivecommunication is utilized for i2i communication, a received signal islikely to have a low amplitude, e.g., be under 1 mV in amplitude.Accordingly, in such a system the use of a traditional differentialamplifier as a receiver is not useful as the input offset voltage of atraditional differential amplifier will typically be greater than 1 mV,e.g., likely 10 mV or more with additional variation over time andtemperature. Additionally, a traditional differential amplifier is verysusceptible to input offset voltage drift, which is undesirable whenattempting to detect low amplitude i2i signals.

SUMMARY

Implantable medical devices (IMDs), and methods for use therewith, aredescribed herein. Such an IMD can be a leadless pacemaker (LP)configured to be implanted within or on a wall of an atrial orventricular chamber, but is not limited thereto.

In accordance with certain embodiments, an IMD includes, inter alia, areceiver and a battery. The receiver includes first and seconddifferential amplifiers, each of which is selectively enabled, each ofwhich includes differential inputs, and each of which includes anoutput. The battery powers electrical components of the IMD, includingthe first and second differential amplifiers, while the electricalcomponents are enabled. Each of the first and second differentialamplifiers, while enabled, is configured to monitor for a predeterminedsignal within a frequency range. The predetermined signal can be, e.g.,a wakeup signal within a frequency range. More specifically, the wakeupsignal can be a low frequency wakeup pulse within a low frequency range,e.g., from 1 kHz to 100 kHz. Each of the first and second differentialamplifiers, while enabled, drains current and thereby power from thebattery. By contrast, each of the first and second differentialamplifiers, while not enabled, drains substantially no current and thussubstantially no power from the battery. Substantially no current andsubstantially no power, as the phrases are used herein, meanrespectively at least 100× less current and at least 100× less powerthan is consumed by a differential amplifier when it is enabled. Inorder to remove undesirable input offset voltages due, e.g., to inputoffset voltage drift, each of the first and second differentialamplifiers, while enabled, is capable of being selectively put into anoffset correction phase during which time the predetermined signal(e.g., a wakeup pulse) is not detectable by the differential amplifier.In accordance with certain embodiments of the present technology, at anygiven time at least one of the first and second differential amplifiersof the receiver is enabled without being in the offset correction phaseso that at least one of the first and second differential amplifiers isalways monitoring for the predetermined signal within the frequencyrange. In this manner, the receiver is never blind to messages,including wakeup signals, sent by another IMD, such as another LP.

In accordance with specific embodiments of the present technology, thefirst and second differential amplifiers are simultaneously enabled forless than 20% of a time that only one of the first and seconddifferential amplifiers is enabled. Additionally, one of the first andsecond differential amplifiers is in the offset correction phase for atleast a majority of a time that the first and second differentialamplifiers are simultaneously enabled. To further save battery power, inaccordance with specific embodiments of the present technology, thefirst and second differential amplifiers are simultaneously enabled forless than 10% the time that only one of the first and seconddifferential amplifiers is enabled.

In accordance with specific embodiments of the present technology, theIMD also includes first and second electrodes that are coupled to thedifferential inputs of each of the first and second differentialamplifiers. In this manner, the electrodes can be used for receivingimplant-to-implant (i2i) communication signals. The electrodes can alsobe used for transmitting i2i communication signals. The same electrodescan also be used for delivering cardiac stimulation pulses, as well asfor sensing intrinsic or evoked cardiac activity.

In accordance with specific embodiments of the present technology, thereceiver including the first and second differential amplifiers is afirst receiver, and the IMD also includes a second receiver that isnominally kept in a disabled state when not being used, in order toconserver power. In such embodiments, the predetermined signal that eachof the first and second differential amplifiers of the first receiver isconfigured to monitor for can be a wakeup signal (e.g., a wakeup pulse)within a first frequency range (e.g., from 1 kHz to 100 kHz). The secondreceiver, which is also coupled to the first and second electrodes, isselectively enabled in response to one of the first and seconddifferential amplifiers of the first receiver detecting the wakeupsignal within the first frequency range. The second receiver isconfigured to receive one or more i2i communication signals within asecond frequency range that is higher than the first frequency rangewhile the second receiver is enabled. The second receiver consumes morepower than the first receiver while the second receiver is enabled, andthus, it is beneficial to keep the second receiver disabled except whenit is needed. In accordance with specific embodiments of the presenttechnology, the IMD includes OR gate circuitry having inputs coupled tothe outputs of the first and second differential amplifiers of the firstreceiver and having an output coupled to an enable terminal of thesecond receiver. Pulse conditioning circuitry is optionally coupledbetween the output of the OR gate circuitry and the enable terminal ofthe second receiver.

In accordance with specific embodiments of the present technology, theIMD is a leadless pacemaker (LP) including a hermetic housing thatsupports the first and second electrodes and within which the first andsecond receivers and the battery are disposed.

In accordance with specific embodiments of the present technology, whilethe first differential amplifier is enabled and in the offset correctionphase, the second differential amplifier is enabled without being in theoffset correction phase. Conversely, while the second differentialamplifier is enabled and in the offset correction phase, the firstdifferential amplifier is enabled without being in the offset correctionphase. This way at least one of the first and second differentialamplifiers is always enabled without being in the offset correctionphase, and thus, is always capable of monitoring for and detecting thepredetermined signal (e.g., the wakeup signal). This is why the receiverthat includes the first and second differential amplifiers can be saidto always be on.

In accordance with specific embodiments of the present technology, eachof the first and second differential amplifiers is an auto-zerodifferential amplifier, and the offset correction phase is an auto-zerophase. Alternatively, each of the first and second differentialamplifiers can be a chopper-stabilized differential amplifier, in whichcase the offset correction phase is a chopper-stabilization phase. Othervariations are also possible and within the scope of the embodimentsdescribed herein.

Certain embodiments of the present technology are related to methods foruse with an implantable medical device (IMD) having a receiver includingfirst and second differential amplifiers, wherein each the first andsecond differential amplifiers includes differential inputs and anoutput, and wherein each of the first and second differential amplifiersis capable of being selectively put in an offset correction phase whileenabled. Such a method can include selectively enabling the first andsecond differential amplifiers such that at any given time at least oneof the first and second differential amplifiers is enabled. Such amethod can also include selectively putting the first and seconddifferential amplifiers in an offset correction phase such that whilethe first differential amplifier is enabled and in the offset correctionphase the second differential amplifier is enabled without being in theoffset correction phase, and such that while the second differentialamplifier is enabled and in the offset correction phase the firstdifferential amplifier is enabled without being in the offset correctionphase. Additionally, such a method can also include always using atleast one of the first and second differential amplifiers to monitor fora predetermined signal (e.g., a wakeup signal) within a frequency range.

In accordance with certain embodiments, in order to conserve power theselectively enabling of the first and second differential amplifiers isperformed such that the first and second differential amplifiers aresimultaneously enabled for less than 20% (and more preferably, for lessthan 10%) of a time that only one of the first and second differentialamplifiers is enabled.

The receiver including the first and second differential amplifiers canbe a first receiver, and the predetermined signal can be a wakeup signalwithin a first frequency range. In such an embodiment, the method canalso include enabling a second receiver in response to the wakeup signalwithin the first frequency range being detected by at least one of thefirst and second differential amplifiers of the first receiver, whereinthe second receiver consumes more power than the first receiver whilethe second receiver is enabled. The method can further include using thesecond receiver to receive one or more implant-to-implant (i2i)communication signals within a second frequency range that is higherthan the first frequency range while the second receiver is enabled.

This summary is not intended to be a complete description of theembodiments of the present technology. Other features and advantages ofthe embodiments of the present technology will appear from the followingdescription in which the preferred embodiments have been set forth indetail, in conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present technology relating to both structure andmethod of operation may best be understood by referring to the followingdescription and accompanying drawings, in which similar referencecharacters denote similar elements throughout the several views:

FIG. 1 illustrates a system formed in accordance with certainembodiments herein as implanted in a heart.

FIG. 2A is a block diagram of a single LP in accordance with certainembodiments herein.

FIG. 2B provides additional details of one of the receivers of the LPintroduced with reference to FIG. 2A, according to certain embodimentsof the present technology.

FIG. 2C is an exemplary timing diagram associated with enable and offsetcorrection signals provided to each of the differential amplifiers ofthe receiver whose details are introduced with reference to FIG. 2B,according to certain embodiments of the present technology.

FIG. 3 illustrates an LP in accordance with certain embodiments herein.

FIG. 4 is a timing diagram demonstrating one embodiment of implant toimplant (i2i) communication for a paced event.

FIG. 5 is a timing diagram demonstrating one embodiment of i2icommunication for a sensed event.

FIG. 6 is high level flow diagram that are used to summarize methodsaccording to certain embodiments of the present technology.

DETAILED DESCRIPTION

As mentioned above, where conductive communication is utilized for i2icommunication, a received signal is likely to have a low amplitude,e.g., be under 1 mV in amplitude. Accordingly, in such a system the useof a traditional differential amplifier is not useful as the inputoffset voltage of the integrated amplifier will typically be greaterthan 1 mV, e.g., likely 10 mV or more with additional variation overtime and temperature. Additionally, a traditional differential amplifieris very susceptible to input offset voltage drift, which is undesirablewhen attempting to detect low amplitude i2i signals.

Instead of using a differential amplifier, a single ended amplifier mayinstead be used. However, this would mandate the use of a very quietpower supply for the receiver since a single ended topology has verypoor power supply rejection ratio. This type of receiver is verysensitive to power supply noise and may falsely trigger due to powersupply and other noise sources.

Instead of using a traditional differential amplifier, or a single endedamplifier, an auto-zeroed differential amplifier could be used removethe input offset voltage issue. However, during the auto-zero time (alsoreferred to as the auto-zero phase) the amplifier would be “blind” toi2i messages, meaning that the auto-zeroed differential amplifier maynot detect an i2i message while being auto-zeroed. This may not beacceptable in a dual chamber pacemaker system, since this may result insynchrony between two implantable medical devices being lost, which mayresult in a dangerous situation.

Certain embodiments of the present technology relate to a redundantparallel fully differential auto-zeroed system which combines the lowinput offset voltage, and thus, the high sensitivity of a low offsetdifferential system but without the time period where the system is“blind” due to the auto-zero function. More generally, certainembodiments of the present invention utilize a pair of differentialinput amplifiers that are selectively enabled in and selectively offsetcorrected in antiphase in a manner that consumes only slightly morecurrent than would be consumed by one differential input amplifier.

Before providing addition details of the specific embodiments of thepresent technology mentioned above, an exemplary system in whichembodiments of the present technology can be used will first bedescribed with reference to FIGS. 1-5. More specifically, FIGS. 1-5 willbe used to describe an exemplary cardiac pacing system, wherein pacingand sensing operations can be performed by multiple medical devices,which may include one or more leadless cardiac pacemakers, an ICD, suchas a subcutaneous-ICD, and/or a programmer reliably and safelycoordinate pacing and/or sensing operations.

FIG. 1 illustrates a system 100 that is configured to be implanted in aheart 101. The system 100 includes two or more leadless pacemakers (LPs)102 and 104 located in different chambers of the heart. LP 102 islocated in a right atrium, while LP 104 is located in a right ventricle.LPs 102 and 104 communicate with one another to inform one another ofvarious local physiologic activities, such as local intrinsic events,local paced events and the like. LPs 102 and 104 may be constructed in asimilar manner, but operate differently based upon which chamber LP 102or 104 is located.

In certain embodiments, LPs 102 and 104 communicate with one another,and/or with an ICD 106, by conductive communication through the sameelectrodes that are used for sensing and/or delivery of pacing therapy.The LPs 102 and 104 may also be able to use conductive communications tocommunicate with an external device, e.g., a programmer 109, havingelectrodes placed on the skin of a patient within with the LPs 102 and104 are implanted. While not shown (and not preferred, since it wouldincrease the size of the LPs 102 and 104), the LPs 102 and 104 canpotentially include an antenna and/or telemetry coil that would enablethem to communicate with one another, the ICD 106 and/or an externaldevice using RF or inductive communication.

In some embodiments, one or more leadless cardiac pacemakers 102 and 104can be co-implanted with the implantable cardioverter-defibrillator(ICD) 106. Each leadless cardiac pacemaker 102, 104 uses two or moreelectrodes located within, on, or within a few centimeters of thehousing of the pacemaker, for pacing and sensing at the cardiac chamber,for bidirectional communication with one another, with the programmer109, and the ICD 106.

In accordance with certain embodiments, devices and methods are providedfor coordinating operation between leadless pacemakers (LPs) located indifferent chambers of the heart. The devices and methods enable a localLP to receive communications from a remote LP through conductivecommunication.

While the methods, devices and systems described herein include examplesprimarily in the context of LPs, it is understood that the methods,devices and systems described herein may be utilized with various otherexternal and implanted devices. By way of example, the methods, devicesand systems may coordinate operation between various implantable medicaldevices (IMDs) implanted in a human, not just LPs. Certain embodimentsenable a first IMD to receive communications from at least a second IMDthrough conductive communication over at least a first channel. Itshould also be understood that the embodiments described herein can beused for communication between multiple IMDs, and are not limited tocommunication between just a first and second IMD. The methods, devicesand systems may also be used for communication between two or more IMDsimplanted within the same chamber that may be the same type of IMD ormay be different types of IMDs. The methods, devices and systems mayalso be used for communication between two or more IMDs in a systemincluding at least one IMD implanted but not within a heart chamber,e.g., epicardially, transmurally, intravascularly (e.g., coronarysinus), subcutaneously (e.g., S-ICD), etc.

Referring to FIG. 2A, a block diagram shows an embodiment for portionsof the electronics within LP 102, 104 configured to provide conductivecommunication through the sensing/pacing electrode. One or more of LPs102 and 104 include at least two leadless electrodes 108 configured fordelivering cardiac pacing pulses, sensing evoked and/or natural cardiacelectrical signals, and uni-directional or bi-directional communication.

LP 102, 104 includes first and second receivers 120 and 122 thatcollectively define separate first and second communication channels 105and 107 (FIG. 1), (among other things) between LPs 102 and 104. Althoughfirst and second receivers 120 and 122 are depicted, in otherembodiments, LP 102, 104 may only include first receiver 120, or mayinclude additional receivers other than first and second receivers 120and 122. As will be described in additional detail below, the pulsegenerator 116 can function as a transmitter that transmits i2icommunication signals using the electrodes 108. In certain embodiments,LPs 102 and 104 may communicate over more than just first and secondcommunication channels 105 and 107. In certain embodiments, LPs 102 and104 may communicate over one common communication channel 105. Morespecifically, LPs 102 and 104 can communicate conductively over a commonphysical channel via the same electrodes 108 that are also used todeliver pacing pulses. Usage of the electrodes 108 for communicationenables the one or more leadless cardiac pacemakers 102 and 104 toperform antenna-less and telemetry coil-less communication.

The receivers 120 and 122 can also be referred to, respectively, as alow frequency (LF) receiver 120 and a high frequency (HF) receiver 122,because the receiver 120 is configured to monitor for one or moresignals within a relatively low frequency range (e.g., below 100 kHz)and the receiver 122 is configured to monitor for one or more signalswithin a relatively high frequency range (e.g., above 100 kHz). Incertain embodiments, the receiver 120 (and more specifically, at least aportion thereof) is always enabled and monitoring for a wakeup notice,which can simply be a wakeup pulse, within a specific low frequencyrange (e.g., between 1 kHz and 100 kHz); and the receiver 122 isselectively enabled by the receiver 120. The receiver 120 is configuredto consume less power than the receiver 122 when both the first andsecond receivers are enabled. Accordingly, the receiver 120 can also bereferred to as a low power receiver 120, and the receiver 122 can alsobe referred to as a high power receiver 122. Additional details of thereceiver 120, according to certain embodiments of the presenttechnology, are described further below with reference to FIGS. 2B and2C. The low power receiver 120 is incapable of receiving signals withinthe relatively high frequency range (e.g., above 100 kHz), but consumessignificantly less power than the high power receiver 122. This way thelow power receiver 120 is capable of always monitoring for a wakeupnotice without significantly depleting the battery (e.g., 114) of theLP. In accordance with certain embodiments, the high power receiver 122is selectively enabled by the low power receiver 120, in response to thelow power receiver 120 receiving a wakeup notice, so that the high powerreceiver 122 can receive the higher frequency signals, and therebyhandle higher data throughput needed for effective i2i communicationswithout unnecessarily and rapidly depleting the battery of the LP (whichthe high power receiver 122 may do if it were always enabled).

In accordance with certain embodiments, when one of the LPs 102 and 104senses an intrinsic event or delivers a paced event, the correspondingLP 102, 104 transmits an implant event message to the other LP 102, 104.For example, when an atrial LP 102 senses/paces an atrial event, theatrial LP 102 transmits an implant event message including an eventmarker indicative of a nature of the event (e.g., intrinsic/sensedatrial event, paced atrial event). When a ventricular LP 104senses/paces a ventricular event, the ventricular LP 104 transmits animplant event message including an event marker indicative of a natureof the event (e.g., intrinsic/sensed ventricular event, pacedventricular event). In certain embodiments, LP 102, 104 transmits animplant event message to the other LP 102, 104 preceding the actual pacepulse so that the remote LP can blank its sense inputs in anticipationof that remote pace pulse (to prevent inappropriate crosstalk sensing).

The implant event messages may be formatted in various manners. As oneexample, each event message may include a leading trigger pulse (alsoreferred to as an LP wakeup notice, wakeup pulse or wakeup signal)followed by an event marker. The notice trigger pulse (also referred toas the wakeup notice, wakeup pulse or wakeup signal) is transmitted overa first channel (e.g., with a pulse duration of approximately 10 μs toapproximately 1 ms and/or within a fundamental frequency range ofapproximately 1 kHz to approximately 100 kHz). The notice trigger pulseindicates that an event marker is about to be transmitted over a secondchannel (e.g., within a higher frequency range). The event marker canthen be transmitted over the second channel.

The event markers may include data indicative of one or more events(e.g., a sensed intrinsic atrial activation for an atrial located LP, asensed intrinsic ventricular activation for a ventricular located LP).The event markers may include different markers for intrinsic and pacedevents. The event markers may also indicate start or end times fortimers (e.g., an AV interval, a blanking interval, etc.). Optionally,the implant event message may include a message segment that includesadditional/secondary information.

Optionally, the LP (or other IMD) that receives any implant to implant(i2i) communication from another LP (or other IMD) or from an externaldevice may transmit a receive acknowledgement indicating that thereceiving LP/IMD received the i2i communication, etc.

The event messages enable the LPs 102, 104 to deliver synchronizedtherapy and additional supportive features (e.g., measurements, etc.).To maintain synchronous therapy, each of the LPs 102 and 104 is madeaware (through the event messages) when an event occurs in the chambercontaining the other LP 102, 104. Some embodiments described hereinprovide efficient and reliable processes to maintain synchronizationbetween LPs 102 and 104 without maintaining continuous communicationbetween LPs 102 and 104. In accordance with certain embodiments herein,low power event messages/signaling may be maintained between LPs 102 and104 synchronously or asynchronously.

For synchronous event signaling, LPs 102 and 104 maintainsynchronization and regularly communicate at a specific interval.Synchronous event signaling allows the transmitter and receivers in eachLP 102,104 to use limited (or minimal) power as each LP 102, 104 is onlypowered for a small fraction of the time in connection with transmissionand reception. For example, LP 102, 104 may transmit/receive (Tx/Rx)communications in time slots having duration of 10-20 μs, where theTx/Rx time slots occur periodically (e.g., every 10-20 ms).

LPs 102 and 104 may lose synchronization, even in a synchronous eventsignaling scheme. As explained herein, features may be included in LPs102 and 104 to maintain device synchronization, and when synchronizationis lost, LPs 102 and 104 undergo operations to recover synchronization.Also, synchronous event messages/signaling may introduce a delay betweentransmissions which causes a reaction lag at the receiving LP 102, 104.Accordingly, features may be implemented to account for the reactionlag.

During asynchronous event signaling, LPs 102 and 104 do not maintaincommunication synchronization. During asynchronous event signaling, oneor more of receivers 120 and 122 of LPs 102 and 104 may be “always on”(always awake) to search for incoming transmissions. However,maintaining LP receivers 120, 122 in an “always on” (always awake) statepresents challenges as the received signal level often is low due tohigh channel attenuation caused by the patient's anatomy. Further,maintaining the receivers awake will deplete the battery 114 morequickly than may be desirable.

The asynchronous event signaling methods avoid risks associated withlosing synchronization between devices. However, the asynchronous eventsignaling methods utilize additional receiver current betweentransmissions. For purposes of illustration only, a non-limiting exampleis described hereafter. For example, the channel attenuation may beestimated to have a gain of 1/500 to 1/10000. A gain factor may be1/1000th. Transmit current is a design factor in addition to receivercurrent. As an example, the system may allocate one-half of the implantcommunication current budget to the transmitter (e.g., 0.5 μA for eachtransmitter). When LP 102, 104 maintains a transmitter in a continuouson-state and the electrode load is 500 ohms, a transmitted voltage maybe 2.5V. When an event signal is transmitted at 2.5V, the event signalis attenuated as it propagates and would appear at LP 102, 104 receiveras an amplitude of approximately 0.25 mV.

To overcome the foregoing receive power limit, a pulsed transmissionscheme may be utilized in which communication transmissions occurcorrelated with an event. By way of example, the pulsed transmissionscheme may be simplified such that each transmission constitutes asingle pulse of a select amplitude and width.

In accordance with certain embodiments herein, LPs 102 and 104 mayutilize multi-stage receivers that implement a staged receiver wakeupscheme in order to improve reliability yet remain power efficient. Eachof LPs 102 and 104 may include first and second receivers 120 and 122that operate with different first and second activation protocols anddifferent first and second receive channels. For example, first receiver120 may be assigned a first activation protocol that is “always on”(also referred to as always awake) and that listens over a first receivechannel that has a lower fundamental frequency range/pulse duration(e.g., 1 kHz to 100 kHz/10 μs to approximately 1 ms) as compared to thefundamental frequency range (e.g., greater than 100 kHz/less than 10 μsper pulse) assigned to the second receive channel.

In accordance with certain embodiments, the first receiver 120 maymaintain the first channel active (awake) at all times (including whenthe second channel is inactive (asleep)) in order to listen for messagesfrom a remote LP. The second receiver 122 may be assigned a secondactivation protocol that is a triggered protocol, in which the secondreceiver 122 becomes active (awake) in response to detection of triggerevents over the first receive channel (e.g., when the incoming signalcorresponds to the LP wakeup notice, activating the second channel atthe local LP). The terms active, awake and enabled are usedinterchangeably herein.

Still referring to FIG. 2A, each LP 102, 104 is shown as including aprocessor or controller 112 and a pulse generator 116. The processor orcontroller 112 can include, e.g., a microprocessor (or equivalentcontrol circuitry), RAM and/or ROM memory, logic and timing circuitry,state machine circuitry, and I/O circuitry, but is not limited thereto.The processor or controller 112 can further include, e.g., timingcontrol circuitry to control the timing of the stimulation pulses (e.g.,pacing rate, atrio-ventricular (AV) delay, atrial interconduction (A-A)delay, or ventricular interconduction (V-V) delay, etc.). Such timingcontrol circuitry may also be used for the timing of refractory periods,blanking intervals, noise detection windows, evoked response windows,alert intervals, marker channel timing, and so on. The processor orcontroller 112 can further include other dedicated circuitry and/orfirmware/software components that assist in monitoring variousconditions of the patient's heart and managing pacing therapies. Theprocessor or controller 112 and the pulse generator 116 may beconfigured to transmit event messages, via the electrodes 108, in amanner that does not inadvertently capture the heart in the chamberwhere LP 102, 104 is located, such as when the associated chamber is notin a refractory state. In addition, a LP 102, 104 that receives an eventmessage may enter an “event refractory” state (or event blanking state)following receipt of the event message. The event refractory/blankingstate may be set to extend for a determined period of time after receiptof an event message in order to avoid the receiving LP 102, 104 frominadvertently sensing another signal as an event message that mightotherwise cause retriggering. For example, the receiving LP 102, 104 maydetect a measurement pulse from another LP 102, 104 or programmer 109.

In accordance with certain embodiments herein, programmer 109 maycommunicate over a programmer-to-LP channel, with LP 102, 104 utilizingthe same communication scheme. The external programmer may listen to theevent message transmitted between LP 102, 104 and synchronize programmerto implant communication such that programmer 109 does not transmitcommunication signals 113 until after an implant to implant messagingsequence is completed.

In accordance with certain embodiments, LP 102, 104 may combine transmitoperations with therapy. The transmit event marker may be configured tohave similar characteristics in amplitude and pulse-width to a pacingpulse and LP 102, 104 may use the energy in the event messages to helpcapture the heart. For example, a pacing pulse may normally be deliveredwith pacing parameters of 2.5V amplitude, 500 ohm impedance, 60 bpmpacing rate, 0.4 ms pulse-width. The foregoing pacing parameterscorrespond to a current draw of about 1.9 μA. The same LP 102, 104 mayimplement an event message utilizing event signaling parameters foramplitude, pulse-width, pulse rate, etc. that correspond to a currentdraw of approximately 0.5 μA for transmit.

LP 102, 104 may combine the event message transmissions with pacingpulses. For example, LP 102, 104 may use a 50 μs wakeup transmit pulsehaving an amplitude of 2.5V which would draw 250 nC (nano Coulombs) foran electrode load of 500 ohm. The pulses of the transmit event messagemay be followed by an event message encoded with a sequence of shortduration pulses (for example 16, 2 μs on/off bits) which would draw anadditional 80 nC. The event message pulse would then be followed by theremaining pulse-width needed to reach an equivalent charge of a nominal0.4 ms pace pulse. In this case, the current necessary to transmit themarker is essentially free as it was used to achieve the necessary pacecapture anyhow. With this method, the savings in transmit current couldbe budgeted for the receiver or would allow for additional longevity.

When LP 102 or 104 senses an intrinsic event, it can send aqualitatively similar event pulse sequence (but indicative of a sensedevent) without adding the pace pulse remainder. As LP 102, 104 longevitycalculations are designed based on the assumption that LP 102, 104 willdeliver pacing therapy 100% of the time, transmitting an intrinsic eventmarker to another LP 102, 104 will not impact the nominal calculated LPlongevity.

In some embodiments, LP 102, 104 may deliver pacing pulses at relativelylow amplitude. When low amplitude pacing pulses are used, the powerbudget for event messages may be modified to be a larger portion of theoverall device energy budget. As the pacing pulse amplitude is loweredcloser to amplitude of event messages, LP 102, 104 increases an extentto which LP 102, 104 uses the event messages as part of the pacingtherapy (also referred to as sharing “capture charge” and “transmitcharge”). As an example, if the nominal pacing voltage can be lowered to<1.25 V, then a “supply halving” pacing charge circuit could reduce thebattery current draw by approximately 50%. A 1.25V pace pulse would save1.5 μA of pacing current budget. With lower pulse amplitudes, LP 102,104 may use larger pulse-widths.

By combining event messages and low power pacing, LP 102, 104 mayrealize additional longevity. Today longevity standards provide that thelongevity to be specified based on a therapy that utilizes 2.5Vamplitude, 0.4 ms pulses at 100% pacing. Optionally, a new standard maybe established based on pacing pulses that deliver lower amplitudeand/or shorter pacing pulses.

While not shown, a communication capacitor can be provided in LP 102,104. The communication capacitor may be used to transmit event signalshaving higher voltage for the event message pulses to improvecommunication, such as when the LPs 102 and 104 experience difficultysensing event messages. The high voltage event signaling may be used forimplants with high signal attenuation or in the case of a retry for anARQ (automatic repeat request) handshaking scheme.

In some embodiments, the individual LP 102 can comprise a hermetichousing 110 configured for placement on or attachment to the inside oroutside of a cardiac chamber and at least two leadless electrodes 108proximal to the housing 110 and configured for bidirectionalcommunication with at least one other device 106 within or outside thebody.

FIG. 2A depicts a single LP 102 (or 104) and shows the LP's functionalelements substantially enclosed in a hermetic housing 110. The LP 102(or 104) has at least two electrodes 108 located within, on, or near thehousing 110, for delivering pacing pulses to and sensing electricalactivity from the muscle of the cardiac chamber, and for bidirectionalcommunication with at least one other device within or outside the body.Hermetic feedthroughs 130, 131 conduct electrode signals through thehousing 110. The housing 110 contains a primary battery 114 to supplypower for pacing, sensing, and communication. The housing 110 alsocontains circuits 132 for sensing cardiac activity from the electrodes108, receivers 120, 122 for receiving information from at least oneother device via the electrodes 108, and the pulse generator 116 forgenerating pacing pulses for delivery via the electrodes 108 and alsofor transmitting information to at least one other device via theelectrodes 108. The housing 110 can further contain circuits formonitoring device health, for example a battery current monitor 136 anda battery voltage monitor 138, and can contain circuits for controllingoperations in a predetermined manner.

The electrodes 108 can be configured to communicate bidirectionallyamong the multiple leadless cardiac pacemakers and/or the implanted ICD106 to coordinate pacing pulse delivery and optionally other therapeuticor diagnostic features using messages that identify an event at anindividual pacemaker originating the message and a pacemaker receivingthe message react as directed by the message depending on the origin ofthe message. An LP 102, 104 that receives the event message reacts asdirected by the event message depending on the message origin orlocation. In some embodiments or conditions, the two or more leadlesselectrodes 108 can be configured to communicate bidirectionally amongthe one or more leadless cardiac pacemakers 102 and/or the ICD 106 andtransmit data including designated codes for events detected or createdby an individual pacemaker. Individual pacemakers can be configured toissue a unique code corresponding to an event type and a location of thesending pacemaker.

In some embodiments, an individual LP 102, 104 can be configured todeliver a pacing pulse with an event message encoded therein, with acode assigned according to pacemaker location and configured to transmita message to one or more other leadless cardiac pacemakers via the eventmessage coded pacing pulse. The pacemaker or pacemakers receiving themessage are adapted to respond to the message in a predetermined mannerdepending on type and location of the event.

Moreover, information communicated on the incoming channel can alsoinclude an event message from another leadless cardiac pacemakersignifying that the other leadless cardiac pacemaker has sensed aheartbeat or has delivered a pacing pulse, and identifies the locationof the other pacemaker. For example, LP 104 may receive and relay anevent message from LP 102 to the programmer. Similarly, informationcommunicated on the outgoing channel can also include a message toanother leadless cardiac pacemaker or pacemakers, or to the ICD, thatthe sending leadless cardiac pacemaker has sensed a heartbeat or hasdelivered a pacing pulse at the location of the sending pacemaker.

Referring again to FIGS. 1 and 2, the cardiac pacing system 100 maycomprise an implantable cardioverter-defibrillator (ICD) 106 in additionto leadless cardiac pacemaker 102, 104 configured for implantation inelectrical contact with a cardiac chamber and for performing cardiacrhythm management functions in combination with the implantable ICD 106.The implantable ICD 106 and the one or more leadless cardiac pacemakers102, 104 configured for leadless intercommunication by informationconduction through body tissue and/or wireless transmission betweentransmitters and receivers in accordance with the discussed herein.

In a further embodiment, a cardiac pacing system 100 comprises at leastone leadless cardiac pacemaker 102, 104 configured for implantation inelectrical contact with a cardiac chamber and configured to performcardiac pacing functions in combination with the co-implantedimplantable cardioverter-defibrillator (ICD) 106. The leadless cardiacpacemaker or pacemakers 102 comprise at least two leadless electrodes108 configured for delivering cardiac pacing pulses, sensing evokedand/or natural cardiac electrical signals, and transmitting informationto the co-implanted ICD 106.

As shown in the illustrative embodiments, a leadless cardiac pacemaker102, 104 can comprise two or more leadless electrodes 108 configured fordelivering cardiac pacing pulses, sensing evoked and/or natural cardiacelectrical signals, and bidirectionally communicating with theco-implanted ICD 106.

LP 102, 104 can be configured for operation in a particular location anda particular functionality at manufacture and/or at programming by anexternal programmer. Bidirectional communication among the multipleleadless cardiac pacemakers can be arranged to communicate notificationof a sensed heartbeat or delivered pacing pulse event and encoding typeand location of the event to another implanted pacemaker or pacemakers.LP 102, 104 receiving the communication decode the information andrespond depending on location of the receiving pacemaker andpredetermined system functionality.

In some embodiments, the LPs 102 and 104 are configured to beimplantable in any chamber of the heart, namely either atrium (RA, LA)or either ventricle (RV, LV). Furthermore, for dual-chamberconfigurations, multiple LPs may be co-implanted (e.g., one in the RAand one in the RV, one in the RV and one in the coronary sinus proximatethe LV). Certain pacemaker parameters and functions depend on (orassume) knowledge of the chamber in which the pacemaker is implanted(and thus with which the LP is interacting; e.g., pacing and/orsensing). Some non-limiting examples include: sensing sensitivity, anevoked response algorithm, use of AF suppression in a local chamber,blanking & refractory periods, etc. Accordingly, each LP needs to knowan identity of the chamber in which the LP is implanted, and processesmay be implemented to automatically identify a local chamber associatedwith each LP.

Processes for chamber identification may also be applied to subcutaneouspacemakers, ICDs, with leads and the like. A device with one or moreimplanted leads, identification and/or confirmation of the chamber intowhich the lead was implanted could be useful in several pertinentscenarios. For example, for a DR or CRT device, automatic identificationand confirmation could mitigate against the possibility of the clinicianinadvertently placing the V lead into the A port of the implantablemedical device, and vice-versa. As another example, for an SR device,automatic identification of implanted chamber could enable the deviceand/or programmer to select and present the proper subset of pacingmodes (e.g., AAI or VVI), and for the IPG to utilize the proper set ofsettings and algorithms (e.g., V-AutoCapture vs ACap-Confirm, sensingsensitivities, etc.).

Also shown in FIG. 2A, the primary battery 114 has positive terminal 140and negative terminal 142. Current from the positive terminal 140 ofprimary battery 114 flows through a shunt 144 to a regulator circuit 146to create a positive voltage supply 148 suitable for powering theremaining circuitry of the pacemaker 102. The shunt 144 enables thebattery current monitor 136 to provide the processor 112 with anindication of battery current drain and indirectly of device health. Theillustrative power supply can be a primary battery 114.

In various embodiments, LP 102, 104 can manage power consumption to drawlimited power from the battery, thereby reducing device volume. Eachcircuit in the system can be designed to avoid large peak currents. Forexample, cardiac pacing can be achieved by discharging a tank capacitor(not shown) across the pacing electrodes. Recharging of the tankcapacitor is typically controlled by a charge pump circuit. In aparticular embodiment, the charge pump circuit is throttled to rechargethe tank capacitor at constant power from the battery.

In some embodiments, the processor or controller 112 in one leadlesscardiac pacemaker 102 can access signals on the electrodes 108 and canexamine output pulse duration from another pacemaker for usage as asignature for determining triggering information validity and, for asignature arriving within predetermined limits, activating delivery of apacing pulse following a predetermined delay of zero or moremilliseconds. The predetermined delay can be preset at manufacture,programmed via an external programmer, or determined by adaptivemonitoring to facilitate recognition of the triggering signal anddiscriminating the triggering signal from noise. In some embodiments orin some conditions, the processor or controller 112 can examine outputpulse waveform from another leadless cardiac pacemaker for usage as asignature for determining triggering information validity and, for asignature arriving within predetermined limits, activating delivery of apacing pulse following a predetermined delay of zero or moremilliseconds.

FIG. 3 shows an LP 102, 104. The LP can include a hermetic housing 202with electrodes 108 a and 108 b disposed thereon. As shown, electrode108 a can be separated from but surrounded partially by a fixationmechanism 205, and the electrode 108 b can be disposed on the housing202. The fixation mechanism 205 can be a fixation helix, a plurality ofhooks, barbs, or other attaching features configured to attach thepacemaker to tissue, such as heart tissue. The electrodes 108 a and 108b are examples of the electrodes 108 shown in and discussed above withreference to FIG. 2A.

The housing can also include an electronics compartment 210 within thehousing that contains the electronic components necessary for operationof the pacemaker, including, e.g., a pulse generator, receiver, abattery, and a processor for operation. The hermetic housing 202 can beadapted to be implanted on or in a human heart, and can be cylindricallyshaped, rectangular, spherical, or any other appropriate shapes, forexample.

The housing can comprise a conductive, biocompatible, inert, andanodically safe material such as titanium, 316L stainless steel, orother similar materials. The housing can further comprise an insulatordisposed on the conductive material to separate electrodes 108 a and 108b. The insulator can be an insulative coating on a portion of thehousing between the electrodes, and can comprise materials such assilicone, polyurethane, parylene, or another biocompatible electricalinsulator commonly used for implantable medical devices. In theembodiment of FIG. 3, a single insulator 208 is disposed along theportion of the housing between electrodes 108 a and 108 b. In someembodiments, the housing itself can comprise an insulator instead of aconductor, such as an alumina ceramic or other similar materials, andthe electrodes can be disposed upon the housing.

As shown in FIG. 3, the pacemaker can further include a header assembly212 to isolate 108 a and 108 b. The header assembly 212 can be made fromPEEK, tecothane or another biocompatible plastic, and can contain aceramic to metal feedthrough, a glass to metal feedthrough, or otherappropriate feedthrough insulator as known in the art.

The electrodes 108 a and 108 b can comprise pace/sense electrodes, orreturn electrodes. A low-polarization coating can be applied to theelectrodes, such as sintered platinum, platinum-iridium, iridium,iridium-oxide, titanium-nitride, carbon, or other materials commonlyused to reduce polarization effects, for example. In FIG. 3, electrode108 a can be a pace/sense electrode and electrode 108 b can be a returnelectrode. The electrode 108 b can be a portion of the conductivehousing 202 that does not include an insulator 208.

Several techniques and structures can be used for attaching the housing202 to the interior or exterior wall of the heart. A helical fixationmechanism 205, can enable insertion of the device endocardially orepicardially through a guiding catheter. A torqueable catheter can beused to rotate the housing and force the fixation device into hearttissue, thus affixing the fixation device (and also the electrode 108 ain FIG. 3) into contact with stimulable tissue. Electrode 108 b canserve as an indifferent electrode for sensing and pacing. The fixationmechanism may be coated partially or in full for electrical insulation,and a steroid-eluting matrix may be included on or near the device tominimize fibrotic reaction, as is known in conventional pacingelectrode-leads.

Implant-to-Implant Event Messaging

LPs 102 and 104 can utilize implant-to-implant (i2i) communicationthrough event messages to coordinate operation with one another invarious manners. The terms i2i communication, i2i event messages, andi2i even markers are used interchangeably herein to refer to eventrelated messages and IMD/IMD operation related messages transmitted froman implanted device and directed to another implanted device (althoughexternal devices, e.g., a programmer, may also receive i2i eventmessages). In certain embodiments, LP 102 and LP 104 operate as twoindependent leadless pacers maintaining beat-to-beat dual-chamberfunctionality via a “Master/Slave” operational configuration. Fordescriptive purposes, the ventricular LP 104 shall be referred to as“vLP” and the atrial LP 102 shall be referred to as “aLP”. LP 102, 104that is designated as the master device (e.g. vLP) may implement all ormost dual-chamber diagnostic and therapy determination algorithms. Forpurposes of the following illustration, it is assumed that the vLP is a“master” device, while the aLP is a “slave” device. Alternatively, theaLP may be designated as the master device, while the vLP may bedesignated as the slave device. The master device orchestrates most orall decision-making and timing determinations (including, for example,rate-response changes).

In accordance with certain embodiments, methods are provided forcoordinating operation between first and second leadless pacemakers(LPs) configured to be implanted entirely within first and secondchambers of the heart. A method transmits an event marker throughconductive communication through electrodes located along a housing ofthe first LP, the event marker indicative of one of a local paced orsensed event. The method detects, over a sensing channel, the eventmarker at the second LP. The method identifies the event marker at thesecond LP based on a predetermined pattern configured to indicate thatan event of interest has occurred in a remote chamber. In response tothe identifying operation, the method initiates a related action in thesecond LP.

FIG. 4 is a timing diagram 400 demonstrating one example of an i2icommunication for a paced event. The i2i communication may betransmitted, for example, from LP 102 to LP 104. As shown in FIG. 4, inthis embodiment, an i2i transmission 402 is sent prior to delivery of apace pulse 404 by the transmitting LP (e.g., LP 102). This enables thereceiving LP (e.g., LP 104) to prepare for the remote delivery of thepace pulse. The i2i transmission 402 includes an envelope 406 that mayinclude one or more individual pulses. For example, in this embodiment,envelope 406 includes a low frequency pulse 408 followed by a highfrequency pulse train 410. Low frequency pulse 408 lasts for a periodTi2iLF, and high frequency pulse train 410 lasts for a period Ti2iHF.The end of low frequency pulse 408 and the beginning of high frequencypulse train 410 are separated by a gap period, Ti2iGap.

As shown in FIG. 4, the i2i transmission 402 lasts for a period Ti2iP,and pace pulse 404 lasts for a period Tpace. The end of i2i transmission402 and the beginning of pace pulse 404 are separated by a delay period,TdelayP. The delay period may be, for example, between approximately 0.0and 10.0 milliseconds (ms), particularly between approximately 0.1 msand 2.0 ms, and more particularly approximately 1.0 ms. The termapproximately, as used herein, means+/−10% of a specified value.

FIG. 5 is a timing diagram 500 demonstrating one example of an i2icommunication for a sensed event. The i2i communication may betransmitted, for example, from LP 102 to LP 104. As shown in FIG. 5, inthis embodiment, the transmitting LP (e.g., LP 102) detects the sensedevent when a sensed intrinsic activation 502 crosses a sense threshold504. A predetermined delay period, TdelayS, after the detection, thetransmitting LP transmits an i2i transmission 506 that lasts apredetermined period Ti2iS. The delay period may be, for example,between approximately 0.0 and 10.0 milliseconds (ms), particularlybetween approximately 0.1 ms and 2.0 ms, and more particularlyapproximately 1.0 ms.

As with i2i transmission 402, i2i transmission 506 may include anenvelope that may include one or more individual pulses. For example,similar to envelope 406, the envelope of i2i transmission 506 mayinclude a low frequency pulse followed by a high frequency pulse train.

Optionally, wherein the first LP is located in an atrium and the secondLP is located in a ventricle, the first LP produces an AS/AP eventmarker to indicate that an atrial sensed (AS) event or atrial paced (AP)event has occurred or will occur in the immediate future. For example,the AS and AP event markers may be transmitted following thecorresponding AS or AP event. Alternatively, the first LP may transmitthe AP event marker slightly prior to delivering an atrial pacing pulse.Alternatively, wherein the first LP is located in an atrium and thesecond LP is located in a ventricle, the second LP initiates anatrioventricular (AV) interval after receiving an AS or AP event markerfrom the first LP; and initiates a post atrial ventricular blanking(PAVB) interval after receiving an AP event marker from the first LP.

Optionally, the first and second LPs may operate in a “pure”master/slave relation, where the master LP delivers “command” markers inaddition to or in place of “event” markers. A command marker directs theslave LP to perform an action such as to deliver a pacing pulse and thelike. For example, when a slave LP is located in an atrium and a masterLP is located in a ventricle, in a pure master/slave relation, the slaveLP delivers an immediate pacing pulse to the atrium when receiving an APcommand marker from the master LP.

In accordance with some embodiments, communication and synchronizationbetween the aLP and vLP is implemented via conducted communication ofmarkers/commands in the event messages (per i2i communication protocol).As explained above, conducted communication can include event messagestransmitted from the sensing/pacing electrodes at frequencies outsidethe RF or Wi-Fi frequency range. The figures and correspondingdescription below illustrate non-limiting examples of markers that maybe transmitted in event messages. The figures and correspondingdescription below also include the description of the markers andexamples of results that occur in the LP that receives the eventmessage. Table 1 represents exemplary event markers sent from the aLP tothe vLP, while Table 2 represents exemplary event markers sent from thevLP to the aLP. In the master/slave configuration, AS event markers aresent from the aLP each time that an atrial event is sensed outside ofthe post ventricular atrial blanking (PVAB) interval or some otheralternatively-defined atrial blanking period. The AP event markers aresent from the aLP each time that the aLP delivers a pacing pulse in theatrium. The aLP may restrict transmission of AS markers, whereby the aLPtransmits AS event markers when atrial events are sensed both outside ofthe PVAB interval and outside the post ventricular atrial refractoryperiod (PVARP) or some other alternatively-defined atrial refractoryperiod. Alternatively, the aLP may not restrict transmission of AS eventmarkers based on the PVARP, but instead transmit the AS event markerevery time an atrial event is sensed.

TABLE 1 “A2V” Markers/Commands (i.e., from aLP to vLP) MarkerDescription Result in vLP AS Notification of a sensed event in InitiateAV interval (if not atrium (if not in PVAB or PVARP) in PVAB or PVARP)AP Notification of a paced event in Initiate PAVB atrium Initiate AVinterval (if not in PVARP)

As shown in Table 1, when an aLP transmits an event message thatincludes an AS event marker (indicating that the aLP sensed an intrinsicatrial event), the vLP initiates an AV interval timer. If the aLPtransmits an AS event marker for all sensed events, then the vLP wouldpreferably first determine that a PVAB or PVARP interval is not activebefore initiating an AV interval timer. If however the aLP transmits anAS event marker only when an intrinsic signal is sensed outside of aPVAB or PVARP interval, then the vLP could initiate the AV intervaltimer upon receiving an AS event marker without first checking the PVABor PVARP status. When the aLP transmits an AP event marker (indicatingthat the aLP delivered or is about to deliver a pace pulse to theatrium), the vLP initiates a PVAB timer and an AV interval time,provided that a PVARP interval is not active. The vLP may also blank itssense amplifiers to prevent possible crosstalk sensing of the remotepace pulse delivered by the aLP.

TABLE 2 “V2A” Markers/Commands (i.e., from vLP to aLP) MarkerDescription Result in aLP VS Notification of a sensed event in InitiatePVARP ventricle VP Notification of a paced event in Initiate PVABventricle Initiate PVARP AP Command to deliver immediate Deliverimmediate pace pulse pace pulse in atrium to atrium

As shown in Table 2, when the vLP senses a ventricular event, the vLPtransmits an event message including a VS event marker, in response towhich the aLP may initiate a PVARP interval timer. When the vLP deliversor is about to deliver a pace pulse in the ventricle, the vLP transmitsVP event marker. When the aLP receives the VP event marker, the aLPinitiates the PVAB interval timer and also the PVARP interval timer. TheaLP may also blank its sense amplifiers to prevent possible crosstalksensing of the remote pace pulse delivered by the vLP. The vLP may alsotransmit an event message containing an AP command marker to command theaLP to deliver an immediate pacing pulse in the atrium upon receipt ofthe command without delay.

The foregoing event markers are examples of a subset of markers that maybe used to enable the aLP and vLP to maintain full dual chamberfunctionality. In one embodiment, the vLP may perform all dual-chamberalgorithms, while the aLP may perform atrial-based hardware-relatedfunctions, such as PVAB, implemented locally within the aLP. In thisembodiment, the aLP is effectively treated as a remote ‘wireless’ atrialpace/sense electrode. In another embodiment, the vLP may perform mostbut not all dual-chamber algorithms, while the aLP may perform a subsetof diagnostic and therapeutic algorithms. In an alternative embodiment,vLP and aLP may equally perform diagnostic and therapeutic algorithms.In certain embodiments, decision responsibilities may be partitionedseparately to one of the aLP or vLP. In other embodiments, decisionresponsibilities may involve joint inputs and responsibilities.

In an embodiment, ventricular-based pace and sense functionalities arenot dependent on any i2i communication, in order to provide safertherapy. For example, in the event that LP to LP (i2i) communication islost (prolonged or transient), the system 100 may automatically revertto safe ventricular-based pace/sense functionalities as the vLP deviceis running all of the necessary algorithms to independently achievethese functionalities. For example, the vLP may revert to a VVI mode asthe vLP does not depend on i2i communication to perform ventricularpace/sense activities. Once i2i communication is restored, the system100 can automatically resume dual-chamber functionalities.

Always on Receiver with Offset Correction

As noted above, in the discussion of FIG. 2A, each of the LPs 102, 104includes first and second receivers 120 and 122, which can also bereferred to respectively as LF and HF receivers 120 and 122, or lowpower and high power receivers 120 and 122. In accordance with certainembodiments, the LF receiver 120 is configured to monitor for a wakeupsignal within a relatively low frequency range (e.g., below 100 kHz),and the receiver 122 (when enabled) is configured to monitor for eventmessage signals within a relatively high frequency range (e.g., above100 kHz). In certain embodiments, the receiver 120 (and morespecifically, at least a portion thereof) is always enabled andmonitoring for a wakeup notice, which can simply be a wakeup pulse,within a specific low frequency range (e.g., between 1 kHz and 100 kHz);and the receiver 122 is selectively enabled by the receiver 120. Thereceiver 120 is configured to consume less power than the receiver 122when both the first and second receivers are enabled. For example, thereceiver 122 may consume about 50 μA when enabled, and the receiver 120may consume on average only about 0.3 μA.

In accordance with certain embodiments, the wakeup signal that thereceiver 120 is configured to monitor for and detect for has a lowamplitude that may be under 1 mV. Accordingly, it would not be useful toimplement the receiver 120 using a traditional differential amplifierbecause the input offset voltage of a traditional differential amplifierwill typically be greater than 1 mV (e.g., likely 10 mV or more withadditional variation over time and temperature). Additionally, atraditional differential amplifier is very susceptible to input offsetvoltage drift, which is undesirable when attempting to detect lowamplitude signals of 1 mV or less.

Instead of using a traditional differential amplifier to implement thereceiver 120, a single ended amplifier may be used. However, this wouldundesirably mandate the use of a very quite power supply for thereceiver 120 since a single ended topology has very poor power supplyrejection ratio. It would be difficult to implement such a quiet powersupply within an IMD, such as the LP 102, 104.

Another option would be to implement the receiver 120 using anauto-zeroed differential amplifier, which could remove the input offsetvoltage issue. However, during its auto-zero phase, the auto-zeroeddifferential amplifier would not be able to detect a signal sent fromanother LP, and thus, would be blind to a wakeup signal sent fromanother LP (or from an external programmer 109). This would not beacceptable in a dual chamber pacemaker system because synchrony betweenthe two LPs 102 and 104 could be lost, resulting in a potentiallydangerous situation.

In accordance with certain embodiments of the present technology, whichinitially will be described with reference to FIG. 2B, two auto-zeroeddifferential amplifiers (or more generally, differential amplifierscapable of being selectively offset corrected) are used in parallel andantiphase to implement the receiver 120. Such embodiments provide forlow input offset voltage, and thus, the high sensitivity of low offsetdifferential system, in a manner that avoids the receiver 120 beingblind to wakeup signals during an auto-zero phase (or more generally, anoffset correction phase), as will be better understood from the belowdiscussion of FIG. 2B.

Referring to FIG. 2B, the receiver 120 is shown as including twodifferential amplifiers 124 a and 124 b, each of which includesdifferential inputs and an output. The differential amplifiers 124 a and124 b can be referred to collectively as the differential amplifiers124, or individually as a differential amplifier 124. Each of thedifferential amplifiers 124 includes a respective enable terminal thatallows each of the differential amplifiers 124 to be selectivelyenabled. Each of the differential amplifiers 124, while enabled, drainscurrent (e.g., ˜300 nA) and thereby power from the battery. Conversely,each of the differential amplifiers 124, while not enabled, drainssubstantially no current (i.e., less than 10 nA) and thus substantiallyno power (i.e., less than 20 nA) from the battery 114. Each of thedifferential amplifiers 124 also includes a respective offset correctionterminal that enables each of the differential amplifiers 124 (whileenabled) to be selectively offset corrected. Depending uponimplementation, each of the differential amplifiers 124 can beconfigured to be enabled in response to the signal at its enableterminal being HIGH (or alternatively, in response to the signal at itsenable terminal being LOW). Depending upon implementation, each of thedifferential amplifiers 124 can be configured to be offset corrected(while enabled) in response to the signal at its offset correctionterminal being HIGH (or alternatively, in response to the signal at itsoffset correction terminal being LOW). When a differential amplifier 124is not enabled, it can also be said that the differential amplifier 124is disabled. When a differential amplifier 124 is enabled and not in itsoffset correction phase (i.e., not being offset corrected), it can besaid that the differential amplifier 124 is in its normal sensing andamplifying phase.

In accordance with certain embodiments, each of the differentialamplifiers 124 is implemented as an auto-zeroed differential amplifier,in which case the offset correction phase is more specifically anauto-zero phase during which the differential amplifier 124 isauto-zeroed to remove an input offset voltage. In accordance withalternative embodiments, each of the differential amplifiers 124 isimplemented as a chopper-stabilized differential amplifier, in whichcase the offset correction phase is more specifically achopper-stabilization phase. Other variations are possible, and withinthe scope of embodiments of the present technology described herein.

In accordance with certain embodiments, each of the differentialamplifiers 124, while enabled, is configured to monitor for a wakeupsignal. As explained above with reference to FIG. 4, the wakeup signalcan be low amplitude (e.g., 1 mV) and low frequency pulse 408 (that isfollowed by a low amplitude and high frequency pulse train 410). For amore specific example, the wakeup signal can be a 1 mV pulse having apulse-width of 20 μs, in which case each of the differential amplifierscan be specifically tuned to monitor for and detect a 1 mV wakeup pulsehaving a 20 μs pulse-width.

The signal that is output from each of the differential amplifiers 124can be referred to as an amplified difference signal. Depending uponimplementation, each of the differential amplifiers 124 can beconfigured to nominally output a LOW amplified difference signal (whenthe wakeup pulse is not detected) and output a HIGH amplified differencesignal in response to the wakeup pulse being detected. Alternatively,although unlikely, each of the differential amplifiers 124 can beconfigured nominally output a HIGH amplified difference signal (when thewakeup pulse is not detected) and output a LOW amplified differencesignal in response to the wakeup pulse being detected. For the remainderof this discussion, it will be assumed that each of the differentialamplifiers 124 is configured nominally output a LOW amplified differencesignal (when the wakeup pulse is not detected) and output a HIGHamplified difference signal in response to the wakeup pulse beingdetected.

During its offset correction phase (e.g., auto-zero phase, or chopperstabilization phase), a differential amplifier 124 is unable to detectthe signal (e.g., the wakeup pulse) that the differential amplifier 124is configured to monitor for, and thus, the amplified difference signalproduced at the output of the differential amplifier 124 will not beindicative of whether or not the predetermined signal (e.g., the wakeuppulse) is detected by the differential amplifier 124. For example, whilea differential amplifier 124 is in its offset correction phase, theamplified difference signal output by the differential amplifier 124 mayhover about halfway between a LOW and HIGH output level. Accordingly, inaccordance with certain embodiments, the amplified difference signaloutput by a differential amplifier 124 is ignored when the differentialamplifier 124 is in its offset correction phase (e.g., its auto-zerophase, or its chopper stabilization phase). In other words, inaccordance with certain embodiments, the output of a differentialamplifier 124 is ignored when the differential amplifier 124 is in itsoffset correction phase.

Still referring to FIG. 2B, the outputs of the differential amplifiers124 a and 124 b are shown as being provided to respective inputs of anOR gate circuit 126, which is used to combine the outputs of thedifferential amplifiers 124 a and 124 b. More specifically, the outputof the OR gate circuit 126 will go HIGH in response to at least one ofthe inputs to the OR gate circuit 126 going HIGH. One of the inputs tothe OR gate circuit 126 will be HIGH when one of the differentialamplifiers 124 a and 124 b detects the wakeup signal (e.g., the wakeuppulse) that the differential amplifiers 124 a and 124 b are eachmonitoring for. Accordingly, the output of the OR gate 126 will go HIGH,and cause the receiver 122 to be enabled, in response to one of thedifferential amplifiers 124 a and 124 b detecting the wakeup signal theyare monitoring for. The output of the OR gate circuit 126 can beprovided directly to the enable terminal of the receiver 122.Alternatively, an optional pulse conditioning circuit 128 is locatedbetween the output of the OR gate circuit 126 and the enable terminal ofthe receiver 122. The pulse conditioning circuit 128 can, for example,stretch the pulse output by the OR gate circuit 126 to a desiredpulse-width. Alternatively, the pulse conditioning circuit 128 can beconfigured to output a pulse of a specific amplitude and pulse-width inresponse to the input of the pulse conditioning circuit 128 going fromLOW to HIGH and remaining HIGH for at least a predetermined period oftime. Other variations are also possible, and within the scope of theembodiments of the present technology. More generally, other ways ofcombining the outputs of the differential amplifiers 124 a and 124 binto one signal, besides using the OR gate circuit 126, are possible andare within the scope of the embodiments of the present technologydescribed herein.

In accordance with certain embodiments of the present technology, at anygiven time at least one of the differential amplifiers 124 a and 124 bof the receiver 120 is enabled without being in the offset correctionphase (e.g., the auto-zero phase, or the chopper stabilization phase).This beneficially enables at least one of the differential amplifiers124 a and 124 b to always be monitoring for the wakeup signal within itsexpected low frequency range. More specifically, in accordance withcertain embodiments of the present technology, each of the differentialamplifiers 124 a and 124 b is periodically offset corrected, duringwhich time the other one of the differential amplifiers 124 a and 124 bis enabled and not being offset corrected and is thereby monitoring forthe wakeup signal. Accordingly, it should be appreciated that there willbe times that both of the differential amplifiers 124 a and 124 b aresimultaneously enabled, and thus, times that both of the differentialamplifiers 124 a and 124 b are draining current and thereby power fromthe battery 114.

In order to substantially minimize the power that the receiver 120drains from the battery 114, the amount of time that both differentialamplifiers 124 a and 124 b are both enabled should be kept low andpreferably minimized. In accordance with certain embodiments, for atleast a majority of time that both differential amplifiers 124 a and 124b are enabled, one of the differential amplifiers 124 a and 124 b isbeing offset corrected. For example, assume that it takes 3 ms for anoffset correction phase to be completed. If the amount of time that bothof the differential amplifiers 124 a and 124 b are both enabled is 4 msper cycle period (e.g., per 100 ms), then for at least a majority oftime that both differential amplifiers 124 a and 124 b are enabled, oneof the differential amplifiers 124 a and 124 b is being offsetcorrected. Preferably, the amount of time that both differentialamplifiers 124 a and 124 b are enabled without either of them beingoffset corrected is substantially minimized, in order to substantiallyminimize the power drawn by the receiver 120 that includes thedifferential amplifiers 124 a and 124 b. In accordance with certainembodiments, the differential amplifiers 124 a and 124 b aresimultaneously enabled for less than 20% (and preferably, less than 10%)of a time that only the differential amplifiers 124 a and 124 b isenabled.

For an example, assume that during each 64 ms cycle period, each of thedifferential amplifiers 124 a and 124 b is selectively enabled for 36ms. Also assume that only one of the differential amplifiers 124 a and124 b is enabled for 28 ms during each 64 ms cycle period, and that bothof the differential amplifiers are enabled for 4 ms during each 64 mscycle period. In this example, during each 64 ms cycle period, thedifferential amplifiers 124 a and 124 b are simultaneously enabled for˜14% of the time that that only one of the differential amplifiers 124 aand 124 b is enabled (i.e., 4 ms/28 ms=0.14286). This would satisfy therequirement that the differential amplifiers 124 a and 124 b aresimultaneously enabled for less than 20% of a time that only one of thedifferential amplifiers 124 a and 124 b is enabled.

FIG. 2C is an exemplary timing diagram for Enable_A and Offset_correct_Asignals that are provided respectively to the enable terminal and theoffset correct terminal of the differential amplifier 124 a, and theEnable_B and Offset_correct_B signals that are provided respectively tothe enable terminal and the offset correct terminal of the differentialamplifier 124 b. In this example, during each 100 ms cycle period eachof the differential amplifiers 124 a and 124 b is selectively enabledfor 54 ms, only one of the differential amplifiers 124 a and 124 b isenabled for 46 ms during each 100 ms cycle period, and both of thedifferential amplifiers are enabled for 4 ms during each 100 ms cycleperiod. Accordingly, in this example, during each 100 ms cycle period,the differential amplifiers 124 a and 124 b are simultaneously enabledfor ˜8.7% of the time that that only one of the differential amplifiers124 a and 124 b is enabled (i.e., 4 ms/46 ms=0.08696). This wouldsatisfy the requirement that the differential amplifiers 124 a and 124 bare simultaneously enabled for less than 20% of a time that only one ofthe differential amplifiers 124 a and 124 b is enabled. Indeed in thisexample, the differential amplifiers 124 a and 124 b are simultaneouslyenabled for less than 10% of the time that only one of the differentialamplifiers 124 a and 124 b is enabled. Preferably, the amount of timethat both differential amplifiers 124 a and 124 b are simultaneouslyenabled is substantially minimized, and will depend at least in partupon how quickly the differential amplifiers 124 are capable of beingoffset corrected, and the length of the cycle periods. The processor orcontroller 112 (in FIG. 2A) can be used to generate the signals shown inFIG. 2C. Additionally, or alternatively, clock circuitry (notspecifically shown) can be used to produce the signals shown in FIG. 2C,potentially under the control of the processor or controller 112. Othervariations are also possible and within the scope of the embodimentsdescribed herein.

In accordance with certain embodiments of the present technology, afterthe receiver 122 is enabled (by the receiver 120, in response to thereceiver 120 detecting the wakeup signal), the receiver 122 can receiveone or more i2i communication signals within a frequency range that ishigher than the frequency range within which the receiver 120 receivedthe wakeup signal. More generally, the receiver 122 once enable canreceive signals within a higher frequency range than the receiver 120 iscapable of receiving signals. The i2i communication signal(s) receivedby the receiver 122 can be provided to the processor or controller 112and used by the processor or controller 122 to start an interval timer(e.g., an AV interval timer) that is used to trigger delivery of apacing pulse via the electrodes 108. Such a pacing pulse can be producedby the pulse generator 116. Other variations are also possible andwithin the scope of the embodiments described herein.

FIG. 6 is high level flow diagram that are used to summarize methodsaccording to certain embodiments of the present technology. Such methodscan be used with an implantable medical device (IMD) having a receiverincluding first and second differential amplifiers, wherein each thefirst and second differential amplifiers includes differential inputsand an output, and wherein each of the first and second differentialamplifiers is capable of being selectively put in an offset correctionphase while enabled. An example of an IMD with which such methods can beperformed is the LP 102, 104 described above with reference to FIGS.2A-2C. More specifically, FIG. 2B, described above, provides exemplarydetails of a receiver 120 including two differential amplifiers 124 aand 124 b that include differential inputs and an output, and that arecapable of being selectively put in an offset correction phase whileenabled.

Referring to FIG. 6, step 602 involves selectively enabling the firstand second differential amplifiers such that at any given time at leastone of the first and second differential amplifiers is enabled. Inaccordance with certain embodiments, examples of which were discussedabove, step 602 is performed such that the first and second differentialamplifiers are simultaneously enabled for less than 20% of a time thatonly one of the first and second differential amplifiers is enabled.

Still referring to FIG. 6, step 604 involves selectively putting thefirst and second differential amplifiers in an offset correction phasesuch that while the first differential amplifier is enabled and in theoffset correction phase the second differential amplifier is enabledwithout being in the offset correction phase, and such that while thesecond differential amplifier is enabled and in the offset correctionphase the first differential amplifier is enabled without being in theoffset correction phase. An exemplary timing diagram that can be used toperform step 604, as well as step 602, is shown in and described abovewith reference to FIG. 2C. In accordance with certain embodiments, eachof the first and second differential amplifiers is an auto-zerodifferential amplifier, in which case the offset correction phase is anauto-zero phase. In accordance with other embodiments, each of the firstand second differential amplifiers is a chopper-stabilized differentialamplifier, in which case the offset correction phase is achopper-stabilization phase.

Step 606 involves always using at least one of the first and seconddifferential amplifiers to monitor for a predetermined signal within afrequency range. In accordance with certain embodiments, thepredetermined signal is a wakeup signal, and more specifically, can be awakeup pulse. In accordance with certain embodiments, the frequencyrange within which the wakeup signal is monitor for is within a lowfrequency range, which can be, e.g., between 1 kHz and 100 kHz, but isnot limited thereto.

Step 608 involves detecting the predetermined signal (e.g., the wakeupsignal) within the frequency range (e.g., the first frequency range)using one of the first and second differential amplifiers of thereceiver.

Step 610 involves enabling another component of the IMD in response tothe predetermined signal being detected within the frequency range byone of the first and second differential amplifiers. In accordance withcertain embodiments, the receiver that includes the first and seconddifferential amplifiers can be a first receiver (e.g., the receiver 120in FIGS. 2A and 2B). Further, as noted above, the predetermined signalthat the differential amplifiers are configured to monitor for anddetect can be a wakeup signal within a first frequency range. In suchembodiments, step 610 can can involve enabling a second receiver (e.g.,the receiver 122 in FIG. 2A) in response to the wakeup signal within thefirst frequency range being detected by at least one of the first andsecond differential amplifiers of the first receiver.

Step 612 involves using the other component, which was enabled at step610, to perform a predetermined function. For example, step 612 caninvolves using the second receiver (e.g., the receiver 122 in FIG. 2A)to detect one or more implant-to-implant (i2i) communication signalswithin a second frequency range that is higher than the first frequencyrange while the second receiver is enabled. As explain above, such asecond receiver (e.g., 122) consumes more power than the first receiver(e.g., 120) while the second receiver is enabled.

Step 614 involves disabling the other component (e.g., the receiver 122in FIG. 2A) after it has been used, at step 612, to perform itspredetermined function. Such disabling at step 614 is performed in orderto conserver the power of the battery within the IMD that is being usedto power the electrical components of the IMD, which components caninclude, among other components, the first and second differentialamplifiers of the first receiver, as well as to power the secondreceiver while it is enabled.

The steps that are described with reference to FIG. 6 need not beperformed in the exact order described, unless the results of one stepare being used by another step or are being used to trigger the start ofanother step. For example, steps 602, 604 and 606, or at least portionsthereof, can be performed in parallel. By contrast, since step 610 isperformed in response to step 608 being performed, step 610 necessarilyoccurs after step 608.

While many of the embodiments of the present technology described abovehave been described as being for use with LP type IMDs, embodiments ofthe present technology can also be used with other types of IMDs besidesan LP. Accordingly, unless specifically limited to use with an LP, theclaims should not be limited to use with LP type IMDs.

It is to be understood that the subject matter described herein is notlimited in its application to the details of construction and thearrangement of components set forth in the description herein orillustrated in the drawings hereof. The subject matter described hereinis capable of other embodiments and of being practiced or of beingcarried out in various ways. Also, it is to be understood that thephraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Further, it is noted that the term “basedon” as used herein, unless stated otherwise, should be interpreted asmeaning based at least in part on, meaning there can be one or moreadditional factors upon which a decision or the like is made. Forexample, if a decision is based on the results of a comparison, thatdecision can also be based on one or more other factors in addition tobeing based on results of the comparison.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the embodiments ofthe present technology without departing from its scope. While thedimensions, types of materials and coatings described herein areintended to define the parameters of the embodiments of the presenttechnology, they are by no means limiting and are exemplary embodiments.Many other embodiments will be apparent to those of skill in the artupon reviewing the above description. The scope of the embodiments ofthe present technology should, therefore, be determined with referenceto the appended claims, along with the full scope of equivalents towhich such claims are entitled. In the appended claims, the terms“including” and “in which” are used as the plain-English equivalents ofthe respective terms “comprising” and “wherein.” Moreover, in thefollowing claims, the terms “first,” “second,” and “third,” etc. areused merely as labels, and are not intended to impose numericalrequirements on their objects. Further, the limitations of the followingclaims are not written in means—plus-function format and are notintended to be interpreted based on 35 U.S.C. § 112(f), unless and untilsuch claim limitations expressly use the phrase “means for” followed bya statement of function void of further structure.

What is claimed is:
 1. An implantable medical device (IMD), comprising:a receiver including first and second differential amplifiers, each ofwhich is selectively enabled, each of which includes differentialinputs, and each of which includes an output; and a battery that powerselectrical components of the IMD, including the first and seconddifferential amplifiers, while the electrical components are enabled;each of the first and second differential amplifiers, while enabled,configured to monitor for a predetermined signal within a frequencyrange; each of the first and second differential amplifiers, whileenabled, draining current and thereby power from the battery; each ofthe first and second differential amplifiers, while not enabled,draining substantially no current and thus substantially no power fromthe battery; and each of the first and second differential amplifiers,while enabled, capable of being selectively put into an offsetcorrection phase during which time the predetermined signal is notdetectable by the differential amplifier; wherein at any given time atleast one of the first and second differential amplifiers of thereceiver is enabled without being in the offset correction phase so thatat least one of the first and second differential amplifiers is alwaysmonitoring for the predetermined signal within the frequency range. 2.The IMD of claim 1, wherein: the first and second differentialamplifiers are simultaneously enabled for less than 20% of a time thatonly one of the first and second differential amplifiers is enabled; andone of the first and second differential amplifiers is in the offsetcorrection phase for at least a majority of a time that the first andsecond differential amplifiers are simultaneously enabled.
 3. The IMD ofclaim 2, wherein: the first and second differential amplifiers aresimultaneously enabled for less than 10% the time that only one of thefirst and second differential amplifiers is enabled.
 4. The IMD of claim1, further comprising: first and second electrodes; wherein thedifferential inputs of each of the first and second differentialamplifiers are coupled to the first and second electrodes.
 5. The IMD ofclaim 4, wherein the receiver including the first and seconddifferential amplifiers comprises a first receiver, and wherein thepredetermined signal that each of the first and second differentialamplifiers of the first receiver is configured to monitor for comprisesa wakeup signal within a first frequency range, and wherein the IMDfurther comprises: a second receiver coupled to the first and secondelectrodes; the second receiver being selectively enabled in response toone of the first and second differential amplifiers of the firstreceiver detecting the wakeup signal within the first frequency range;the second receiver configured to receive one or more implant-to-implant(i2i) communication signals within a second frequency range that ishigher than the first frequency range while the second receiver isenabled; and the second receiver consuming more power than the firstreceiver while the second receiver is enabled.
 6. The IMD of claim 5,wherein the IMD comprises a leadless pacemaker including a hermetichousing that supports the first and second electrodes and within whichthe first and second receivers and the battery are disposed.
 7. The IMDof claim 5, further comprising OR gate circuitry having inputs coupledto the outputs of the first and second differential amplifiers of thefirst receiver and having an output coupled to an enable terminal of thesecond receiver, and wherein pulse conditioning circuitry is optionallycoupled between the output of the OR gate circuitry and the enableterminal of the second receiver.
 8. The IMD of claim 1, wherein: whilethe first differential amplifier is enabled and in the offset correctionphase, the second differential amplifier is enabled without being in theoffset correction phase; and while the second differential amplifier isenabled and in the offset correction phase, the first differentialamplifier is enabled without being in the offset correction phase. 9.The IMD of claim 1, wherein each of the first and second differentialamplifiers comprises an auto-zero differential amplifier, and whereinthe offset correction phase comprises an auto-zero phase.
 10. The IMD ofclaim 1, wherein each of the first and second differential amplifierscomprises a chopper-stabilized differential amplifier, and wherein theoffset correction phase comprises a chopper-stabilization phase.
 11. Amethod for use with an implantable medical device (IMD) comprising areceiver including first and second differential amplifiers, whereineach the first and second differential amplifiers includes differentialinputs and an output, and wherein each of the first and seconddifferential amplifiers is capable of being selectively put in an offsetcorrection phase while enabled, the method comprising: selectivelyenabling the first and second differential amplifiers such that at anygiven time at least one of the first and second differential amplifiersis enabled; selectively putting the first and second differentialamplifiers in an offset correction phase such that while the firstdifferential amplifier is enabled and in the offset correction phase thesecond differential amplifier is enabled without being in the offsetcorrection phase, and such that while the second differential amplifieris enabled and in the offset correction phase the first differentialamplifier is enabled without being in the offset correction phase; andalways using at least one of the first and second differentialamplifiers to monitor for a predetermined signal within a frequencyrange.
 12. The method of claim 11, wherein the selectively enabling isperformed such that the first and second differential amplifiers aresimultaneously enabled for less than 20% of a time that only one of thefirst and second differential amplifiers is enabled.
 13. The method ofclaim 11, wherein each of the first and second differential amplifierscomprises an auto-zero differential amplifier, and wherein the offsetcorrection phase comprises an auto-zero phase.
 14. The method of claim11, wherein each of the first and second differential amplifierscomprises a chopper-stabilized differential amplifier, and wherein theoffset correction phase comprises a chopper-stabilization phase.
 15. Themethod of claim 11, wherein the receiver including the first and seconddifferential amplifiers comprises a first receiver, and wherein thepredetermined signal comprises a wakeup signal within a first frequencyrange, and the method further comprising: enabling a second receiver inresponse to the wakeup signal within the first frequency range beingdetected by one of the first and second differential amplifiers of thefirst receiver; and using the second receiver to receive one or moreimplant-to-implant (i2i) communication signals within a second frequencyrange that is higher than the first frequency range while the secondreceiver is enabled; wherein the second receiver consumes more powerthan the first receiver while the second receiver is enabled.
 16. Animplantable medical device (IMD), comprising: a first differentialamplifier including first differential inputs and a first output atwhich a first amplified difference signal is output; and a seconddifferential amplifier including second differential inputs and a secondoutput at which a second amplified difference signal is output; whereinthe first and second differential amplifiers are enabled and auto-zeroedsuch that at all times at least one of the the first and seconddifferential amplifiers is enabled and not being auto-zeroed; andwherein at any given time one of the first and second amplifieddifference signals is used to selectively enable a component of the IMDin response to one of the the first and second differential amplifiers,which is enabled and not being auto-zeroed, detecting a predeterminedsignal within a frequency range.
 17. The IMD of claim 16, wherein: thefirst and second differential amplifiers are simultaneously enabled forless than 20% of a time that only one of the first and seconddifferential amplifiers is enabled; and one of the first and seconddifferential amplifiers is being auto-zeroed for at least a majority ofa time that the first and second differential amplifiers aresimultaneously enabled.
 18. The IMD of claim 16, wherein: the first andsecond differential amplifiers are parts of a first receiver of the IMD;and the component of the IMD that is enabled in response to thepredetermined signal being detected comprises a second receiver of theIMD that consumes more power while enabled than the first receiver. 19.The IMD of claim 18, wherein the IMD comprises a cardiac stimulationdevice and the differential inputs of each of the first and seconddifferential amplifiers are connected to electrodes of the cardiacstimulation device, the electrodes for use in selectively deliveringcardiac stimulation and also for use in sensing conductive communicationsignals transmitted by at least one of a further IMD or an externalprogrammer.
 20. The IMD of claim 19, wherein the predetermined signalthat the first and second differential amplifiers are configured tomonitor for comprises a wakeup signal that is sent by a further IMDprior to the further IMD sending an event message in response to whichthe IMD initiates an interval timer used to trigger delivery of a pacingpulse via the electrodes.
 21. The IMD of claim 20, wherein the IMD isconfigured to be implanted within or on a wall of an atrial chamber andthe further IMD from which the wakeup signal and event message isreceived is configured to be implanted within or on a wall of aventricular chamber.